Structural elements for aircraft, especially aircraft doors, conventionally comprise a smooth outer skin, which is provided on the inner side with profiles, ribs and the like riveted or adhesively bonded to it, in order to achieve great stiffness and strength along with low weight.
The production of structural elements of this type is complex, since the stiffening profiles and ribs are often produced from solid material with a degree of machining of over 95%, and the riveting with closely set rivets also requires considerable labor.
The machining of the profiles and ribs from solid material is necessary in order for the final strength of the aluminum or magnesium alloy used not to be impaired by the effect of heat. For the same reason, the connecting of the individual elements takes place by riveting or adhesive bonding and not by welding.
Although this conventional method of production allows structural elements of minimized weight and with adequate strength to be produced, there is no assurance that stress or fatigue cracks will not form in the region of highly loaded riveted connections or regions with high notch stresses.
The invention is based on the problem of providing structural elements for aircraft, especially aircraft doors, which can be produced with low weight and high strength at low cost and, on account of a reduced risk of crack propagation, are easy to repair and service.
Taking this problem as a starting point, a structural element for an aircraft, especially an aircraft door, is proposed, produced according to the invention in a single-piece, integral cast embodiment of an aluminum or magnesium alloy.
Modern low-pressure sand casting processes in particular allow the production of large single-piece, integral cast parts with uniform, thin wall thicknesses which do not require any, or considerably less, machining than the conventional structures assembled from individual elements. To improve the material properties, zircon sand may be used for the molds and cores, if appropriate in conjunction with semipermanent molds.
The structural element may preferably comprise hollow, arched-bridge-like transverse members, longitudinal ribs connecting the feet of the arches of the transverse members and sloping side faces connecting and closing the end faces of the transverse members, and have receptacles with gripping surfaces for straightening and machining. In addition, the structural element, especially in the embodiment as an aircraft door, may be provided with a peripheral flange, stiffened by a peripheral rib.
The structural element may in this case preferably be provided with a smooth outer skin which is cast on as a single piece and integrally, but alternatively may also be adhesively attached or riveted on.
Furthermore, the structural element may be provided with ribs which run approximately parallel to the rib at the flange and extend from the ends of one side face to the ends of the parallel other side face at a distance from the outer feet of the arches of the outer transverse members and approximately perpendicular to the flange, and stiffening triangles running from the feet of the arches to the ribs may be provided.
If the outer skin is cast on as a single piece and integrally, the flange may form an edge region of the outer skin.
In the case of the structural element according to the invention, accumulations of material and rough cross-sectional transitions can be avoided, with unavoidable cross-sectional widenings preferably being arranged in the direction of feeding.
The arched-bridge-like formation of the hollow transverse members allows adequately large openings for core removal to be provided; undercut core geometries are not required. The structural element has no deep, thin slits, from which the core material is difficult to remove, and is provided with adequately large corner radii, facilitating shaking out.
The structural element may preferably be provided with a smooth outer skin, inwardly protruding, parallel longitudinal ribs, hollow elevated transverse members, reaching over the longitudinal ribs in an arched-bridge-like manner, side faces of the same height connecting and closing the end faces of the transverse members, a peripheral flange, forming part of the outer face and stiffened by a peripheral, inwardly projecting rib, and receptacles with gripping surfaces for straightening, adjustment and machining.
With the structural element according to the invention, simple mold parting lines with few cores, adequate drafts, adequate core cross sections for core production and handling and also appropriate core mountings can be achieved.
If the edges of the holes of the arched-bridge-like transverse members are stabilized with beads, a slight distortion of the structural element occurs during a heat treatment following the casting operation. For a heat treatment which can be carried out without any problems, rib structures with low internal stresses are used, rough cross-sectional transitions are avoided and notch stresses are reduced by means of adequately large corner radii. The distortion of the structural element remains low if air quenching is used during the heat treatment.
For cleaning the cast structural elements, all the mold and core parting lines are easily accessible, no crossing cores are used, cleaning ribs are provided in the region of the parting joints to avoid the surface of the workpiece being removed during cleaning, and as many cleaning regions as possible are arranged where machining is subsequently carried out.
Machining is performed in particular on the outer face of the outer skin and/or the inwardly directed surfaces of the transverse members and/or the outer faces of the side faces connecting the end faces of the transverse members and/or the inner face of the peripheral flange and/or the stiffening rib at the flange, on both sides, and/or the gripping surfaces of the receptacles.
If the machining allowances for the outer skin are approximately 6 mm for a machined wall thickness of approximately 1.5 to 2.7 mm and, for the remaining faces to be machined, approximately 3 mm for a machined wall thickness of approximately 3 to 4.2 mm, and the wall thickness of the cast, unmachined regions is 3 to 4.2 mm, a structural element can be produced in a single-piece, integral cast embodiment of a weight which is less than that of a conventionally produced structural element and the production and machining effort for which is likewise greatly reduced.
The receptacles with the gripping surfaces are preferably arranged on dimensionally stable regions, in particular at the four corners of a rectangle formed by the outer transverse members and the side faces connecting them.